This invention relates to echo control in communication channels and, more particularly, to the effective suppressing of echoes in a 2-way telephone circuit at the beginning of a call when the connection is first established.
Echo cancelers of the self-adaptive type are used to provide an echo replica for subtraction from the returning signal. The lower cost of echo cancelers is due to integrated circuit implementations and this has greatly increased the number of communications channels equipped with echo cancelers. In general, the performance of echo cancelers has proven to be desirable for effective elimination of the effect of echo during transmission. However, when a call is first established the echo cancelers have not had sufficient time to become appropriately adapted, also called converged, and may produce a high level spurious output signal, because it is not a true echo replica by virtue of coupling significant energy from the transmission signal back to the returning transmission path.
Subjectively this spurious echo may not actually be objectionable to users of the system due to its relatively short duration. However, other problems are caused by it. For example, during signaling such spurious responses from echo cancelers in a communication system may trigger false operation, or interfere with signaling. Also during automatic maintenance testing, the spurious echo will provide a false indication of the status of the circuits being tested.